Hard X-Ray Spectrum of the Vela Pulsar and Its Wind Nebula Constrained by NuSTAR

Abstract

We present the analysis of 200 ks NuSTAR observation of the Vela pulsar and the pulsar wind nebula (PWN). The phase-resolved spectra corresponding to two main peaks in the folded pulse profile differ significantly. The spectrum of Peak 1 is significantly harder than that of Peak 2 in qualitative agreement with the earlier Rossi X-ray Timing Explorer results. However, for both spectra, the values of power-law (PL) fit photon indices, Γ, are noticeably larger than the previously reported values. The harder (Peak 1) spectrum has Γ = 1.10 ± 0.15, which is close to those measured for the bright inner jets of the PWN. We used the off-pulse interval to remove the emission from the pulsar and measure the compact PWN spectrum in hard X-rays. We also measured the spectrum from the southwestern region of the PWN, which is resolved by NuSTAR from the compact PWN. For both regions, we fit the NuSTAR spectra by themselves and together with the Chandra X-ray Observatory spectra. We found that the PWN spectrum (for both regions) requires a more complex model than a simple PL. The fits to compact PWN spectrum favor an exponentially cutoff PL model, with Ec ≈ 50 keV over the broken PL model. The observed synchrotron photon energies imply electrons accelerated to ≈150 TeV.